Various optical equipment have been used as ophthalmological equipment. Among the equipment, the following optical equipment have been utilized to examine an eye:
Anterior imaging equipment
Fundus camera
Scanning Laser Ophthalmoscope (SLO)
Optical coherence tomography (OCT) utilizing light interference due to low coherent light and other equipment have been used.
Particularly, the optical coherence tomography utilizing light interference because of low coherent light is equipment that is used to acquire a high-resolution fundus tomographic image and has been essential equipment in a specialized retina out-patient clinic. Hereafter, such an optical coherence tomography is described as OCT equipment in short.
The OCT equipment is the equipment with an interferometer to measure with high sensitivity reflected light from a sample by irradiating the sample represented by the retina with low coherent light. Further, the OCT equipment can obtain a tomographic image by scanning the low coherent light over the sample. Specifically, a retina tomographic image is utilized extensively for ophthalmologic diagnosis.
An image with three-dimensional structure can be acquired by continuously acquiring multiple tomographic images when tomographic images are shot by the OCT equipment. However, the time for acquiring the image with a three-dimensional structure becomes long compared to that for acquiring one tomographic image because of shooting the images by scanning multiple tomographic images. Therefore, the image with a three-dimensional structure that is to be acquired must be distorted or displaced when an eyeball moves during the shooting. An eyeball of a subject being tested keeps moving with involuntary movement of the eyeball, so called “involuntary eye movement” even if he/she attempts to see one point. To solve these problems, an OCT proposed in Patent Laid-Open No. 2007-130403 (hereafter, Document 1) corrects a shifted image by aligning an image produced by integrating three dimensional tomographic images in depth direction of a fundus with a two dimensional image of the fundus surface which is acquired by an external means.
As described above, Document 1 shows that if the examined eye moves while shooting an retina image with three dimensional structure, the shifted image is corrected by aligning it with the fundus surface image. However, it is to be hoped that the OCT equipment could prevent the shifted image with high accuracy as it has been highly desirable to have high resolution and high repeatability. With regard to eye movement, there is not only a linear eyeball movement in two directions through the line-of-sight but also circumnutating of rotating about a visual axis. That is, an eyeball has three degrees of freedom. Document 1 does not describe the eyeball movement in three-degrees of freedom as described above every time each of the tomographic images is acquired (scanned).